PVC heat stabilizers is one of the indispensable main processing aids, small parts PVC heat stabilizers used, but its effect is enormous. Use PVC heat stabilizers in PVC processing guarantees not susceptible to degradation, is relatively stable. PVC processing heat stabilizers commonly used basic lead salt stabilizer, a metal soap stabilizers, organic tin stabilizers, rare earth stabilizers, epoxy compounds and the like. PVC complex degradation mechanisms, different mechanism of action is not the same stabilizing agent, the stabilizing effect achieved is also somewhat different.

1. The mechanism of thermal degradation of PVC

PVC at 100 ~ 150 ℃ significant breakdown, UV, mechanical force, oxygen, ozone, hydrogen chloride and some active metal salts and metal oxides will greatly accelerate the decomposition of PVC. PVC thermal oxidative aging complex, some of the literature of the thermal degradation of PVC is divided into two steps. (A) dehydrochlorination: PVC polymer chains off the active chlorine atoms to produce hydrogen chloride, while generating polyene; forming (ii) longer-chain olefins and aromatic multi-ring: With further degradation, allyl chlorine atoms on the extremely unstable and easy off, generate longer chain polyene, the so-called zipper dehydrogenation, while a small amount of CC bond cleavage, cyclization, a small amount of aromatic compounds . Wherein the hydrogen chloride is freed points lead PVC aging main reason. On PVC degradation mechanism is more complex, there is no unified conclusion, there [4] radical mechanism, single-ion mechanism and molecular mechanism proposed by the researchers.

2. PVC heat stabilizers mechanism of

In the process, PVC thermal decomposition is not changed for other properties, mainly affecting the color of the finished product, the thermal stabilizer may be added to inhibit the initial color product. When stripped of HCl mass fraction of 0.1%, PVC color began to change. According to the formation of conjugated double bonds number of different, PVC will show different colors (yellow, orange, red, brown, black). If the PVC during thermal decomposition presence of oxygen available would be colloidal carbon, generating peroxide, a carbonyl group and an ester group compound. But in the long term use of the product, PVC thermal degradation of great influence on the material properties, adding PVC heat stabilizers can delay or reduce the degree of degradation time of PVC degradation.

Thermal stabilizer was added in the processing of PVC The PVC degradation can be suppressed, the main role is played thermal stabilizers are: by substitution of the labile chlorine atoms, the absorption of hydrogen chloride, the addition reaction with unsaturated sites, etc. inhibition degradation of PVC molecules. Thermal stabilizer should preferably have multiple functions:

(1) Replacement lively, labile substituent group such as a chlorine atom attached to the tertiary carbon atoms or an allyl chloride, to form a stable structure;

(2) to absorb and neutralize PVC processing evolved HCl, to eliminate the HCl autocatalytic degradation;

(3) and the passivation or degradation catalytic metal ions and other harmful impurities;

(4) through various forms of chemical reactions can be blocked unsaturated bond continues to grow inhibit the degradation coloring;

(5) UV protective shield for the best effect.

3. PVC stabilizer mechanism and use

3.1 lead salt stabilizer

Lead salt stabilizer [7] can be divided into three categories: (1) pure lead salt stabilizer, mostly containing PbO of salt salt; (2)

Thermal stabilizers with lubricating properties, mainly the fatty acid salt and a neutral salt; solid (3) of composite lead salt stabilizer, and the other containing a lead salt stabilizer component and a synergistic mixture of liquid compound and stabilizer . Lead salt stabilizer heat stability is strong, has good dielectric properties, and low prices, and make a reasonable ratio lubricants PVC resin processing temperature range is widened, processing and product quality and stability after processing, is the most usual stabilizers. Lead salt stabilizer is mainly used in hard products. Lead salt stabilizer has good heat stabilizer, excellent electrical properties, low cost and so on. But the toxic lead salt, can not be used in contact with food products, but also can not obtain a transparent product, and easily sulfide pollution generated black lead sulfide.

3.2 Metal soap stabilizers

Stearic acid soap heat stabilizers are generally alkaline earth metal (calcium, cadmium, zinc, barium, etc.) and stearic acid, lauric saponification system. More types of products, each with its own characteristics. Generally superior lubricity lauric acid, and compatibility with PVC than lauric acid.

Since the metal soap can absorb HCl, some species but also through its catalytic metal ions to replace the root of a fatty acid active sites Cl atoms and may therefore play a thermal stability of PVC in varying degrees. PVC industry is rarely simple metal soap compound, and usually several composite metallic soap. Common are calcium and zinc soap stabilizer. According to Frye-horst mechanism, calcium / zinc compound stabilizer stabilization mechanism can be considered: First, allyl zinc soap and PVC chain reaction of chlorine and calcium soap, zinc soap chloride, chlorine reacts with volatile metal chlorides. In this case, as an intermediary auxiliary stabilizer and then transferred to a chlorine atom calcium soap in order to enable regeneration of zinc soap, delayed generating promote dehydrochlorination of zinc chloride. Ca-Zn stabilizer can be used as nontoxic stabilizers used in food packaging and medical devices, pharmaceutical packaging, but its stability is relatively low teaching, calcium-based stabilizers large amount of poor transparency, easy to bloom. Ca-Zn stabilizer usually used polyol and an antioxidant to improve its performance, the country has a transparent calcium zinc composite stabilizer used in rigid pipe appears.

3.3 organotin stabilizer

Organic tin alkyl tin usually three kinds of methyl, n-butyl, n-octyl and the like. Japanese production mostly butyl tin, Europe octyl tin is more common, which is the European standard approved non-toxic stabilizers, United States methyl tin with relatively more. Common organic tin stabilizer has three categories: (1) aliphatic acid salts, mainly refers to dibutyl tin dilaurate, di-n-octyl tin; (2) maleate class, mainly refers to a horse to dibutyl tin, bis (monobutyl maleate), dibutyltin di-n-octyl tin maleate and the like; (3) salts of mercaptans, wherein the bis (thio carboxylate) ester is the largest amount. Preferably organic tin heat stabilizer performance class, is a rigid PVC products and a good variety of transparent products, especially octyl tin almost become non-toxic packaging products indispensable stabilizer, but its price is more expensive. Organotin heat stabilizers (tin thioglycolate) of PVC have a good stabilizing effect. Especially organotin stabilizer liquid, compared to the solid heat stabilizers, organotin stabilizers can better mixing of the liquid with the PVC resin. Organotin stabilizer (thioglycolate tin) can be substituted with Cl atom unstable polymer, making PVC resins have long-term stability and initial color retention. And proposed stabilization mechanism thioglycolate tin:

(1) S atoms may be substituted with labile Cl atoms, thereby suppressing the generation of conjugated multiolefin.

(2) HCl as a product of the thermal degradation of PVC, and can accelerate polyene formation. And thioglycolic acid can absorb tin produced HCl.

3.4 Rare Earth Stabilizer

Rare earth heat stabilizer include resource-rich light rare earth lanthanum, cerium, neodymium salts of weak organic and inorganic salts. Kind of weak organic acid salt stearic rare earth, rare fatty acid, salicylic acid, rare earth, rare earth citric acid, lauric acid rare earth, rare earth octoate. Mechanism of rare earth stabilizer preliminary study:

Special electronic structure (1) rare earth lanthanide (two outermost electrons, eight times the outer electron structure, there are many unoccupied molecular orbital) of the decision, its unoccupied molecular orbital energy difference is very small, the role of oxygen in the ambient heat or in the role of polar groups, or sub-outer outer electrons are intensified, with the PVC chain unstable Cl ligands, and may break out PVC processing and hydrogen chloride to form coordination complexes, and rare earth elements between chlorine and strong appeal, can play a role in the control of free chlorine, which can prevent or delay the automatic chain reaction of oxidation of hydrogen chloride, thermal stability play a role.

(2) rare earth ionic impurities may multifunctional stabilizer in PVC processing oxygen and PVC itself contains physical adsorption hole and into the crystal lattice of rare earth stabilizer multi-functional, avoiding their parent C-Cl bond shock and vibration. Thus, by the action of the rare earth multifunctional stabilizer can improve the HCl elimination of PVC activation energy, thereby delaying the thermal degradation of PVC plastic.

(3) rare earth compounds suitable anionic groups can play replacement of allyl chloride atom PVC macromolecules on the role of the elimination of the degradation of weakness, but also to achieve stability purposes. RE stabilizer domestic research more. Overall, the stabilizing effect of rare earth heat stabilizer is better than metal soap stabilizers, with good long-term thermal stability, a wide range of synergies between and with other types of stabilizers, with good tolerability, without sulfur pollution, storage-stable, non-toxic and environmentally friendly advantages. In addition, rare earth elements and has a unique role in coupling CaCO3, PVC plasticized with promoting effect, and thus can increase the amount of CaCO3, to reduce the use of processing aids ACR, reduces costs. Stabilizing effect of rare earth on PVC is characterized by its unique synergy. And some rare earth metal, ligand and appropriate with co-stabilizers, can greatly improve the stability.

3.5 Other stabilizers

3.5.1 epoxy

Epoxidized soybean oil, epoxidized linseed oil, epoxidized tall oil can, epoxy butyl stearate, octyl epoxy compounds are commonly used in PVC heat stabilizers deputy, they said stabilizer complexes use a higher synergy advantages of having a light stability and non-toxic, suitable for soft, especially to be exposed to soft FVC article under the sun, not normally used in rigid PVC products, the drawback is easy to bleed . Studies have pointed out, adding to the epoxy sunflower oil containing a different metal soaps (Ba / Cd and Ca / Zn) PVC by measuring the thermal stability of the material and found that sunflower oil and metal soaps have good synergy that enhances the thermal stability of PVC material, analyzes the causes of synergies generated: HCl is absorbed by the degradation of sunflower oil and metal soaps, HCl concentration is reduced while reducing the rate of HCl elimination of PVC (HCl on PVC catalyzes degradation), improves the thermal stability of PVC.

3.5.2 Multi-hydroxysteroid

Pentaerythritol, xylitol and other polyols are of PVC has a certain heat stability, it is commonly used in PVC heat stabilizers deputy. By dehydrochlorination rate and thermal stability experiments and found free of heavy metals and zinc-based heat stabilizers for PVC / polyol thermal stabilization time was extended to 200 ℃, its stabilizing effect of the type and number of hydroxyl groups of the polyol, in particular to a polyhydroxy compound containing hydroxyl end position to promote long term thermal stability PVC, HCl generated during the absorption degradation.

3.5.3 Other

Phosphites, [beta] diketone, dihydropyrimidine, etc. can be used as an auxiliary PVC heat stabilizers, absorbing the produced HCl, retard discoloration of PVC. The current situation and development trend of 4 PVC heat stabilizers for the 21st century, due to global environmental protection requirements increasingly stringent regulations limiting heavy metal stabilizers increasing, so that production and consumption of heat stabilizers to further non-toxic, low toxicity composite efficient direction, lead, cadmium has attracted widespread attention in developed countries, emerging alternative products and applications, lead, cadmium (in particular cadmium) application stabilizers shown a gradual decline in the trend, there are some no toxic or low toxic thermal stabilizers (such as organic tin compounds, calcium and zinc soaps, rare earth stabilizer). Although China in recent years of complex, non-toxic and low toxic heat stabilizer production and development has made considerable achievements, but compared with the world advanced level there are many more deficiencies and gaps (such as less variety, small-scale production, etc.) . China is new heat stabilizer production can not meet the development and application of domestic PVC industry, some of the more high-end PVC products desired heat stabilizer is also largely dependent on imports. The rapid development of China is PVC industry for the development of heat stabilizer industry provides a good market security and broad space for development, but also on the heat stabilizer industry has put forward higher requirements. New heat stabilizer strengthen our research and development, we should pay attention to the following points:

(A) to enhance research and improve the existing lead-free cadmium-free calcium zinc stabilizers, improve the quality of existing products;

(B) According to sources of raw materials and distribution market, and gradually establish the relative concentration of large-scale production plant auxiliary group;

(III) complexes of other PVC additives development and production, the development of multi composite products, to further reduce the waste of resources and environmental pollution, promote sustainable development of "green" aid industry.